Hydrodynamic analysis of drill string in open water
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Due to a decline in proven petroleum reserves through continued production, exploration of oil and gas has moved into more challenging environment and into deep and ultra-deep water. Over the years, the equipment and facilities needed for exploration in such environments have become more sophisticated, expensive and the technology has become more challenging. This is as a result of the need to accommodate the much heavier weight of the marine riser and its suspension/tensioning systems, the space needed for handling more and advanced equipment, the space required for the mud facilities especially when high density drilling fluid is required and finally, to meet standards set by regulating organization. One of the challenges of deep water operation is therefore the marine riser. A marine riser connects the drilling rig to the mudline and helps for material circulation between the well and the topside. In some past years when exploration was done in relatively shallow waters it was easy to move into a deeper waters by only increasing the size of the drill rig. As we move into deeper water, the marine riser and its content get bigger and heavier. Because of the heavier weight larger vessels which are very scares and expensive are required. Therefore, a method that can eliminate the marine riser would be one of attractive alternatives to explore oil fields in deep and ultra-deep water economically. This drilling method without the marine riser is known as Riserless drilling. Riserless drilling is an unconventional drilling technique where the marine riser is eliminated and replaced with a different method to take the return and a control bundle. The control bundle serves as a means of communication between the well and the rig. Reelwell Drilling Method – Riserless (RDM-R) is one of such riserless drilling methods. The RDM-R method utilizes a dual pipe where the return of the circulation fluid is the inner pipe. The company that offers the RDM-R method claims that the benefits of the method includes: reduction in the numbers of casing points - Longer sections, less space requirement, dual gradient drilling, easier station keeping, less weight capacity requirement, less fluid volume and pumping capacity, etc. This report will focus on the effects of hydrodynamic forces and vessel movements on the RDM-R drill string without the marine riser. Emphasis will be made upon the forces and stresses developed during the static and the dynamic phases of the simulation. This report will discuss oscillating loading and the Magnus Effect on the drill string. It will also define for the audience fatigue damage analysis. The RDM-R pipe will be checked for burst and collapse failure with reference to DNV-OS-F201. In this report we will also make comparison of the axial load requirements for a RDM-R method and the CRD method which will be an input for the drill vessel requirements. The input data are referenced from a project undertaken by the Reelwell AS Company in Santos Basin, Brazil. Orcaflex Software will be used for the design of the mechanical model and for the simulation.
Master's thesis in Offshore technology